This invention relates generally to programmable logic devices (PLD), and more particularly the invention relates to a PLD having an efficient programmable macrocell structure which allows greater flexibility in operating on functions and thus higher performance and logic capabilities.
U S. Pat. No. 4,124,899 discloses a programmable array logic circuit in which an array of AND gates is connected to an array of OR gates with inputs to the AND gates being programmable to provide a desired logical output form the OR gates. The outputs from the OR gates can be connected through functionally limited macrocell output circuitry whereby OR outputs can be latched, fed back to the AND array, or connected to I/O pins. Additionally, the I/O pins can be utilized as inputs to the AND array.
More recently, a number of PLD's have been commercially available with both of the AND gate array and OR gate array being programmable, and in which the output macrocell has more versatility in handling data from the gate array, thereby increasing the logic capabilities of the devices. For example, the AMD PAL devices (based on the designs disclosed in U.S. Pat. No. 4,124,899, supra) incorporate multiple clock choices, a flexible output-enable, and feedback selection. The Exel XL8C800 device has dual synchronous and asynchronous logic paths, thereby allowing use of a JK flip-flop while the I/O pin is used as an asynchronous input or output. The ICT PEEL 18CV8 device has an output macro which consists of a D-type flip-flop and two signal selectors which allow twelve possible configurations for the macrocell.
Architectures of most conventional PLD structures that incorporate programmable output functions or I/O macrocells utilize one term that can generate a single sum-of-products function for an input to a register, as feedback to the array, and/or an output to an I/O pin. Other terms, typically product terms, are used to control a variety of I/O macrocell functions such as output-enable or register clock, preset or clear, but the terms are dedicated for a single function. None of the devices allows more than one term, capable of sum-of-products functions, to be programmed to serve multiple functions (i.e. register inputs, register controls, feedback or output). Most of the macrocells have programmable configuration bits which allow combinatorial or sequential functions to be implemented. However, a limiting factor with many of the devices is the inoperability of the entire sum-of-product function when the associated I/O pin of the macrocell is being used as a dedicated input. Some devices, such as the MMI 32VX10, have dual feedbacks that overcome the dedicated input issue, thereby allowing a sequential or combinatorial function to be fed back into the array without negating the use of the I/O pin. However, such devices do not provide output buried configurations within each I/O pin associated macrocell.